Postural Preparation for Sequential Perturbations

The study addresses a question: How are anticipatory postural adjustments (APAs) organized in tasks involving two sequential perturbations that come at a short delay? Two possibilities were compared, a linear superposition of APAs associated with each of the perturbations vs. the generation of an APA related to the overall mechanical effect of both perturbations. Standing subjects performed three tasks involving releasing a load (release-only), catching a load (catch-only), and both in a sequence (release-catch). The load mass and release height co-varied to manipulate different mechanical characteristics of the perturbation. APA magnitude was assessed using integrals of muscle activity over typical time intervals. The APAs associated with load release were smaller for release-catch tasks compared to release-only tasks. The APAs associated with catch were smaller in the trunk and leg muscles and larger in forearm muscles for release-catch tasks compared to the catch-only tasks. APAs were sensitive to the flight time as well as the mass of the load. We conclude that APAs associated with each perturbation were generated taking into account the mechanical effects of the other perturbation and/or the effects of APAs associated with the other perturbation. The findings suggest that studies of relative timing of sequential actions may need to consider associated postural adjustments as factors that may shape the performance. They also offer a possibility that difficulties in sequential actions in certain neurological patients may be causally related to their postural impairment.